Annulenes are the family
of completely conjugated, monocyclic hydrocarbons.
The annulenes are named as [n]-annulene
where n is an even number that represents the number of C atoms in the ring.
We have already met the first
3 members of the series, [4]-, [6]-, and [8]-annulene but we called them 1,3-cyclobutadiene,
benzene and 1,3,5,7-cyclooctatetraene respectively.
Higher members of the annulene
series have also been synthesised and investigated in relation their aromaticity.
Remember that according to the Huckel rule, (4n + 2) π electrons systems are expected to aromatic whereas
4n π electrons
systems are not.
[10]- and [14]-annulene are
not particularly stable due to geometric factors (look at the JSMOL images below).
[16]-annulene is non-planar,
with alternating C=C (average 134 pm) and C-C (average 146
pm) like a non-aromatic polyene.
[10]-annulene
[14]-annulene
[16]-annulene
[18]-annulene
[18]-annulene is almost planar,
with CC bonds between 137-143 pm and an estimated resonance energy
of 418kJ/mol (100kcal/mol)
suggesting some aromatic stability. However, it reacts more like a
polyene than benzene : it undergoes addition reaction with H_{2 }and
Br_{2}, and a Diels-Alder with maleic anhydride.
QUESTION
How do [10]-, [14]- and [18]-annulene relate
to the Huckel rule ? ANSWER